Vegetated coastlines, including seagrass, mangroves, and salt marshes, are valued for their capacity to sequester and store large amounts of organic carbon in their soils.
However, coastal habitats are degrading globally, raising fears that blue carbon habitats could largely disappear by the end of this century unless significant protection and restoration efforts are enacted.
The widespread conversion of Scotland’s saltmarshes to agricultural and development land, especially during the 18th and 19th centuries, together with more recent acceleration of sea-level rise, places this important coastal habitat under extreme pressures.
- There is significant potential for managed realignment in Scotland.
- However, the rates at which additional carbon stores are accumulating are not clear at these sites nor is their potential to contribute GHG emissions back to the atmosphere.
- We found evidence supporting a perceived growing threat of rising sea-levels, particularly associated with the potential loss of saltmarsh area, and associated soil carbon.
- Managed realignment and the creation of new saltmarsh offers a net gain at most sites over the existing saltmarsh area.
- The lower saltmarsh edge only extends down to the mean high water neap level in a few cases. This means local monitoring of vegetation in relation to tidal variation prior to restoration is needed to model future marsh extent.
- To fully assess marsh vulnerability to relative sea-level rise, evidence suggests inclusion of high-tide levels.
It is still not possible to estimate the time taken for a managed realignment site to reach a stable state with natural rates of carbon sequestration. The approach taken in this study to estimate potential blue carbon gains assumes that the realigned saltmarsh will reach a state where it buries and stores organic carbon in a similar way to a natural saltmarsh but does not include this time-dependent process.
Detailed monitoring of existing restored sites across Scotland would improve our understanding of the additional blue carbon gains in soil profiles at these sites, and to also understand the extent of any losses, including GHG emissions, over periods of time.